This invention relates to an electrodeposition process using ultrafiltration in which at least a part of the filtrate is returned to the bath.
U.S. Pat. Nos. 4,136,070 - Hazan (Jan. 23, 1979) and 4,115,227 - Hazan (Sept. 19, 1978), respectively, are directed to coating compositions and aqueous processes for cationic electrodeposition of paints. Typically, a layer of zinc phosphate forms directly on the substrate, followed by a layer of a polymer which is adequately hydrophobic to provide a high-solids content in the initially formed coating. Phosphoric acid is used as an acidic ionizing agent in the bath, and dissolved dihydrogen phosphate salts of metals such as zinc, iron, calcium, magnesium and aluminum are present in the bath and are precipitated by a boundary layer pH change on the substrate as insoluble phosphate salts with either one hydrogen or no hydrogens in the salt.
Several U.S. patents describe ultrafiltration processes particularly adapted for use with electrodeposition. For instance, U.S. Pat. No. 3,663,401 - Christenson et al. (1972) uses ultrafiltration to remove water-soluble acids from an anionic electrodeposition bath. U.S. Pat. Nos. 3,663,404 - Loop (1972) and 3,663,397 - Bras et al. (1972) return directly to the electrodeposition bath at least a portion of the concentrate or retentate and use at least a portion of the filtrate as a rinse for the coated substrate, either remote from the bath or over the bath. Also, informative in this technology are U.S. Pat. Nos. 3,663,400 - Christenson et al. (1972) and 3,663,405 - Christenson et al. (1972).
All of the above-mentioned patents are incorporated herein by reference.
Ultrafiltration is desirable in cationic electrodeposition to control the level of acid and other water-soluble materials in the electrodeposition bath. The filtrate can be disposed of or reused. As an alternative, flushed anode technology can be used for the separation and disposal of acid generated during the operation of a cathodic system by disposal of the anolyte in which the acid is relatively concentrated. Ultrafiltration is useful in initially conditioning an electrocoating bath and in isolating problems. For instance, if too much water is put into the bath along with some of the ingredients, it can be removed by sending the filtrate to a drain. In loading a fresh bath, it is often desirable to send the filtrate of an ultrafilter to drain for one or two turnovers of the bath, to assure optimum operating conditions. Even after a bath has been optimized, excess build-up of acid in cationic bath must be avoided.
It is desirable to be able to operate the process of the two Hazan patents mentioned above, using ultrafiltration, without the need for flushed anodes, while still conserving the phosphoric acid values in the system, thus eliminating or minimizing the need for disposal of filtrate or anolyte and minimizing cost and pollutants.